Fourteen men, aged 20-26 years, were exposed to defined muscular work with a bicycle ergometer. Arterial and venous blood samples were taken during a prework period with the subjects relaxed, during ergometric work at different, but defined watt levels, and during a subsequent period of recovery. The following data were obtained PO2 [O2 pressure], PCO2 [CO2 pressure], pH, standard bi carbonate, base excess, and lactate and pyruvate concentrations. Even during maximal work load (200 watts) femoral PVO2 [venous O2 tension] did not fall below 21.7 [plus or minus] 3.18 mm Hg. Thus the so-called "critical O2 pressure" was never reached. The opinion that the supply of 02 to the tissue of a working muscle is the limiting factor for sustained muscular work is questionable. Femoral venous blood pH was lower than arterial pH in all cases, both during and after ergometric work. However, the fall in venous blood pH was more pronounced than that in arterial blood since, in addition to the increasing levels of fixed acids, there is a considerable rise infemoral PVO2 [venous CO2 tension). Thus, alterations of pH in blood coming from working muscles are not due solely to changes in lactate concentration, as can be assumed for arterial blood. During muscular work and the recovery period, values of standard bicarbonate and base excess correspond to concentrations of lactate and pyruvate; the values in femoral venous blood are lower than in arterial blood, due to the higher concentrations of lactate, pyruvate, and NEFA [nonesterified fatty acids].